Adds expense tracking module

Adds a new module for tracking user expenses.

The module includes:
- Configuration settings for the LNbits API endpoint and timeouts.
- An ExpensesAPI service for fetching accounts and submitting expense entries.
- A UI component for adding expenses, including account selection and form input.
- Dependency injection for the ExpensesAPI service.

This allows users to submit expense entries with account selection and reference data, which will be linked to their wallet.

src/core/di-container.ts

@@ -160,6 +160,9 @@ export const SERVICE_TOKENS = {
 
   // Image upload services
   IMAGE_UPLOAD_SERVICE: Symbol('imageUploadService'),
+
+  // Expenses services
+  EXPENSES_API: Symbol('expensesAPI'),
 } as const
 
 // Type-safe injection helpers